Heat treatment of metals



June 13, 1967 P. GALMICHE 3,325,313

HEAT TREATMENT OF METALS Filed Feb. 24. 1965 INVEN TOR 3/10/ 1 GAL/MUM United States Patent 3 Claims. (Cl. 148-16) This application is a continuation-in-part of my application Ser. No. 42,437 filed July 12, 1960, now abandoned which was itself a division of my prior application Ser. No. 367,286 filed July 10, 1953, now abandoned, for Heat Treatment of Metals.

The present invention relates to the treatment of ferrous metallic pieces which are quenched in order to increase or to regenerate their mechanical properties.

Its object is to perform this treatment in such manner as to avoid any oxidizing of the pieces.

For this purpose, according to the present invention, the treatment comprises performing the heating of the ferrous pieces at a temperature, above the critical range, and for a time corresponding to the prescriptions of the art of quenching, in a chromizing box filled with a chromizing atmosphere containing chromium fluoride and hydrogen and cooling said box, still filled exclusively with said atmosphere, at a rate of cooling such that the rate of cooling of the ferrous pieces in the box correspond to the prescriptions of the quenching art.

Of course, during the heating step, the ferrous pieces, being located in the atmosphere of the box, cannot be oxidized, and, during the cooling step, said pieces are protected against the action of oxygen by the fact that they remain in this atmosphere in the box.

Cooling of the box containing the pieces can be performed either by contact of said box with the free atmosphere or by the action of a cooling liquid on the box, which is either dipped in said liquid (which may be either stationary or constantly renewed by running), or sprinkled therewith.

Preferred embodiments of my invention will be hereinafter described With reference to the accompanying drawing, given merely by way of example, and in which:

The only figure diagrammatically shows in section a system for treating pieces in accordance with my invention.

I shall examine the case of a method of treating ferrous pieces provided with a diffusion superficial layer of chromium obtained by a chromizing treatment having taken place at a temperature and for a time such that the physical and mechanical properties of the pieces are modified.

In this case, this chromizing treatment is first carried out, in a manner which is known in itself, in an atmosphere containing chromium fluoride and hydrogen, advantageously at a temperature of about 1000-1100 C., but a substantial modification of the physical and mechanical properties of the pieces generally has taken place. It is desired to regenerate said properties at least partly.

My invention is particularly interesting when it is desired not only to subject the pieces to a treatment for improving their physical and mechanical properties, but to maintain the superficial state (brightness) which has been given thereto by the chromizing treatment.

The method according to the present invention is of particular interest when the pieces chromized in the above mentioned atmosphere contain substantial amounts of carbon (for instance more than 0.2%

3,325,313 Patented June 13, 1967 In this case, if steps have been taken, in the course of the chromizing operation, to avoid decarburizing, and in particular for steels which contain tungsten, chromizing may have produced a considerably superficial hardening resulting from the formation of a continuous thin superficial layer consisting chiefly of chromium carbide and very hard and of dull appearance.

But the chromizing operation may also be conducted in such manner as to include a superficial decarburizing so as to obtain on the pieces a thick bright layer of chromium diffusion alloy.

The temperature of the chromizing treatment depends upon the nature of the pieces:

Phosphorus pig iron is generally treated at temperatures ranging from 875 to 900 C., and steel containing from 0.4 to 0.6 of carbon is covered with a very hard protective coating of about of a millimeter of chromium carbide after a treatment of two hours at 1075 C. in an atmosphere of chromium fluoride and hydrogen.

If the heating which takes place before quenching is eifected at temperatures ranging from 800 to 900 C., there is practically no chromizing. The atmosphere contains a relatively low amount of active fluoride, the temperature is too low for substantial chromizing and the pieces remain only for a short time at the maximum temperature. It is then unnecessary to provide a reserve of chromium in the treatment chamber because there are practically no losses due to chromium diffusion into said pieces.

In the example illustrated by the drawing, the pieces were placed in a box I normally located in a bell-shaped furnace 6. The reducing atmosphere in box 1 was fed through a conduit C.

Under furnace 6, there was a tank 3 filled with a quenching liquid.

Box 1 was slidable in furnace 6 so that it could be lowered sudenly into the quenching liquid of tank 3.

Conduit C communicated with the clearance space between box 1 and furnace 6 so that the reducing atmosphere fed through conduit C also surrounded box 1.

In order to avoid any inflow of air or steam into the space where the reducing atmosphere was circulating a diaphragm 7 was provided above tank 3, this diaphragm 7 being provided with an opening just suflicient for the passage of treatment box 1 therethrough.

Of course, I may also cool the boxes by spraying them with a cooling liquid.

The rate of cooling of the pieces being substantially lower than that obtained by direct quenching in a liquid, it is advantageous, when it is desired to obtain the best possible mechanical characteristics, to make use of steels having a rate of hardening smaller than the natural rate of cooling of the pieces in air (such steels containing active elements which delay transformation, such in particular as nickel and chromium). An example of composition of such a steel, which is well adapted to a chromizing treatment with accelerated cooling, is the following one: carbon from 0.3 to 0.4%, nickel 4%, chromium 1.2%, silicon from 0.3% to 0.4%, manganese from 0.3 to 0.5%, with possible traces of boron, the remainder being iron and unavoidable impurities.

What I claim is:

1. A method of quenching ferrous metal pieces having a diffused superficial layer of chromium which comprises heating said pieces to a chromizing temperature above the quenching critical range for a time complying with the quenching requirement according to the art, this heating being conducted in a chromizing treatment box filled with a chromizing atmosphere containing hydrogen and chromium fluoride, and subsequently cooling said box in such 3 4 manner that the rate of cooling of said pieces present there- References Cited in is according to the art of quenching, the above men UNITED STATES PATENTS tioned atmosphere being maintained in said box during 2,219,004 10/1940 Daeves et a1. 117107.2 the cooling operation, whereby said p1eces have a non- 2 219005 10/1940 Daeves et a1 2 oxidized surface after said heating and quenching. 5 u

2. A method according to claim 1 wherein said cooling FOREIGN PATENTS of the box is effected by lowering it into a quenching 778,745 7/1957 Great Britain. bath.

3. A method according to claim 1 wherein said cooling HYLAND BIZOT, Primary Examiner of the box is effected by spraying it with a liquid. 10 c LOVELL, Assistant Examinen 

1. A METHOD OF QUENCHING FERROUS METAL PIECES HAVING A DIFFUSED SUPERFICIAL LAYER OF CHROMIUM WHICH COMPRISES HEATING SAID PIECES TO A CHROMIZING TEMPERATURE ABOVE THE QUENCHING CRITICAL RANGE FOR A TIME COMPLYING WITH THE QUENCHING REQUIREMENT ACCORDING TO THE ART, THIS HEATING BEING CONDUCTED IN A CHROMIZING TREATMENT BOX FILLED WITH A CHROMIZING ATMOSPHERE CONTAINING HYDROGEN AND CHROMIUM FLUORIDE, AND SUBSEQUENTLY COOLING SAID BOX IN SUCH MANNER THAT THE RATE OF COOLING OF SAID PIECES PRESENT THEREIN IS ACCORDING TO THE ART OF QUENCHING, THE ABOVE MENTIONED ATMOSPHERE BEING MAINTAINED IN SAID BOX DURING THE COOLING OPERATION, WHEREBY SAID PIECES HAVE A NONOXIDIZED SURFACE AFTER SAID HEATING AND QUENCHING. 